1. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention discloses a method for determining a range of sequence number values for protocol data units that can be retransmitted to a receiver without causing confusion.
2. Description of the Prior Art
In current protocol standards for wireless communications, such as is found in the 3rd Generation Partnership Project (3GPP(trademark)), Technical Specification Group Radio Access Network, RLC Protocol Specification, layer two protocol data units (PDUs) are sent by a transmitter to a receiver. These PDUs have a format that is well defined by the communications protocol, and which includes a sequence number (SN) whose purpose is described below. PDUs are used to carry either layer two signaling information, or layer 3 data. Please refer to FIG. 1. FIG. 1 is a simplified block diagram of a layer 2 PDU 10. The layer 2 PDU 10 includes a 12-bit SN entry 12, and data 14. The actual internal data structure of the PDU 10 can be quite complicated, but for the purposes of the present invention only the SN entry 12 is of importance. The data 14 may hold layer 2 signaling information, layer 3 data, or a combination of the two, and is not of direct relevance to the present invention.
Please refer to FIG. 2 in conjunction with FIG. 1. FIG. 2 is a block diagram of a wireless communications system 20. The wireless communications system 20 includes a receiver 24 and a transmitter 25. The transmitter 25 sends the PDU 10 to the receiver 24. The wireless environment can be quite noisy, and the receiver 24 may not properly receive the PDU 10. The SN entry 12 within the PDU 10 is used by the receiver 24 to determine if all transmitted PDUs 10 have been properly received. For each PDU 10 holding successive data 14, the transmitter 25 increments the value held within the SN entry 12. By analyzing the SN entries 12, the receiver 24 is able to determine which, if any, PDUs 10 were missed, and may subsequently request the transmitter 25 to retransmit the missed PDUs 10. For example, the transmitter 25 may have a large block of data that the transmitter 25 breaks into 4 PDUs 10, which are then successively transmitted to the receiver 24. The first PDU 10 may have an SN entry 12 with a value of 92. The next PDU 10 would be transmitted with an SN entry 12 of 93, followed by the third PDU 10 with an SN entry 12 of 94, and the last with an SN entry 12 of 95. By analyzing these successive values of the SN entries 12, the receiver 24 is able to reconstruct the data sent by the transmitter 25. If any PDUs 10 are found to be missing, the receiver 24 may explicitly inform the transmitter 25 of which PDUs 10 are to be retransmitted.
Both the receiver 24 and the transmitter 25 have windows within which they expect to receive the PDUs 10 and transmit the PDUs 10, respectively. The receiver 24 has a receiving window 21 that is delimited by two state variables: VR(R) 22, and VR(MR) 23. VR(R) 22 marks the beginning of the receiving window 21, and VR(MR) 23 marks the end of the receiving window 21. The receiver 24 will only accept PDUs 10 that have SN entries 12 that are sequentially on or after VR(R) 22 and sequentially before VR(MR) 23. The SN value held in VR(MR) 23 is not considered to be within the receiving window 21. Similarly, the transmitter 25 has a transmitting window 26 that is delimited by two state variables: VT(A) 27 and VT(MS) 28. VT(A) 27 marks the beginning of the transmitting window 26, and VT(MS) 28 marks the end of the transmitting window 26. The transmitter 25 will only transmit PDUs 10 that have SN entries 12 that are within the range of the transmitting window 26, i.e., that are sequentially on or after VT(A) 27, and sequentially before VT(MS) 28.
The fixed length of the SN entry 12, being 12 bits wide, can lead to confusion as to how to treat sequentiality of the PDUs 10. The SN values 12 of the PDUs 10 have a limited range from zero to 4095, after which the SN values 12 rollover back to zero. Because of this, a PDU 10 with an SN value 12 of four, as an example, may be sequentially after a PDU 10 with an SN value 12 of 4092. Care must be taken when considering the SN values 12 to determine their sequential significance.
The receiving window 21 has a fixed receiving window size. The receiving window size is simply the number of SN values spanned by the state variables VR(R) 22 and VR(MR) 23. That is, VR(MR) 23 is always kept a fixed SN value distance away from VR(R) 22, which may be represented mathematically as:
VR(MR)=VR(R)+receiving window size xe2x80x83xe2x80x83(1)
Note that equation (1) is a true 12-bit addition, and will suffer from the rollover discussed above. Thus, VR(MR) 23 does not always contain a value that is numerically larger than VR(R) 22. Similarly, the transmitting window 26 has a transmitting window size, VT(WS) 26a, which indicates the number of SN values spanned by the state variables VT(A) 27 and VT(MS) 28. VT(WS) 26a has an initial value that is set to a configured transmitting window size, which is supplied by layer 3. As above, this may be represented mathematically as:
VT(MS)=VT(A)+VT(WS) xe2x80x83xe2x80x83(2)
And again, the result from equation (2) may suffer from rollover. The receiver 24 may explicitly request the transmitter 25 to change the value of VT(WS) 26a. The requested value of VT(WS) 26a cannot be greater than the configured transmitting window size.
As the receiver 24 receives PDUs 10 from the transmitter 25, the receiver 24 will update that value of the state variable VR(R) 22 to reflect the sequentially earliest SN value 12 before which all preceding PDUs 10 have been successfully received. Put another way, VR(R) 22 always holds the SN value 12 of the sequentially earliest PDU 10 that the receiver 24 is waiting to receive. Upon the successful reception of this PDU 10, the receiver 24 advances the state variable VR(R) 22 to the SN value 12 of the next PDU 10 that needs to be received, and the state variable VR(MR) 23 is updated using equation (1) accordingly . In this manner, the receiving window 21 is advanced by the receiver 24 as the PDUs 10 stream in from the transmitter 25. It should also be noted that the transmitter 25 may explicitly request the receiver 24 to advance the receiving window 21 with a layer 2 signaling PDU, but this has no bearing on the present invention.
The transmitting window 26 is advanced when the transmitter 25 receives a layer 2 status PDU from the receiver 24. The layer 2 status PDU holds the most current value of the state variable VR(R) 22, and is sent at periodic intervals by the receiver 24, or in response to an explicit request from the transmitter 25. The transmitter 25 will then set the state variable VT(A) 27 equal to the value held in the status PDU, which in effect sets VT(A) 27 equal to VR(R) 22. The transmitter 25 updates the state variable VT(MS) using equation (2) accordingly. In this manner, the transmitting window 26 and the receiving window 21 move forward with each other in lock step.
The transmitter 25 has an additional state variable VT(S) 29. When the transmitter 25 begins transmitting the PDUs 10 that lie within the transmitting window 26, the transmitter 25 begins with a PDU 10 having an SN value 12 given by the state variable VT(A) 27, and works sequentially forward until it reaches a PDU 10 having an SN value 12 that is equal to one SN prior to VT(MS) 28. That is, the transmitter 25 transmits the PDUs 10 in sequence, beginning at VT(A) 27 and ending at VT(MS)xe2x88x921. The state variable VT(S) 29 holds the SN value of the next PDU 10 to be transmitted. Thus, the PDUs with SN values on or sequentially after VT(A), and on or sequentially before VT(S)xe2x88x921 have been transmitted at least one time, and are stored in a retransmission buffer 26b until they are acknowledged by the receiver 24. Note that if a PDU with an SN value equal to VT(A) 27 is acknowledged, VT(A) 27 is updated to the next sequentially earliest SN value within the retransmission buffer 26b. 
To increase the probability that the receiving window 21 advances, it is desirable that sequentially early PDUs 10 be retransmitted when retransmission is needed. Unfortunately, the prior art method does not allow the transmitter 25 to capriciously retransmit any PDUs 10 within the retransmission buffer 26b. In fact, in the prior art model, the transmitter 25 may only retransmit PDUs 10 that have been indicated by the receiver 24 as missing. This is due to the vague modulus of the cycling of SN values 12 on the receiver side. That is, it is possible for the receiving window 21 to have advanced up to VT(S) without the transmitter 25 being so informed. If the receiving window 21 is sufficiently wide, and if the transmitter 25 were to retransmit PDUs 10 that had sequentially early SN values 12, it becomes possible for the receiver 24 to mistake old, retransmitted PDUs 10 for new data. Consequently, an exception to this rule is for PDUs 10 having an SN value 12 equal to VT(S)xe2x88x921, which may always be retransmitted. Additionally, the transmitter 25 may retransmit all PDUs 10 within the retransmission buffer 26b if the initial configured transmitting window size is less than 2048, i.e., less than {fraction (212/2)}, half the maximum possible value for SN entries 12. Only under these exceptions may the transmitter 25 retransmit PDUs 10. This, however, is a stringent limit on the allowed retransmission range of PDUs 10, which can adversely affect the efficiency of PDU 10 transportation in the communications system 20. 
It is therefore a primary objective of this invention to provide a method for determining an allowed retransmission range of sequence numbers (SNs) in a communications protocol.
Briefly summarized, the method of the present invention discloses a retransmission range of n-bit sized sequence numbers in a wireless communications system. The wireless communications system has a receiver and a transmitter. The receiver has a receiving window having a receiving window size. The receiving window size indicates the number of sequence numbers spanned by the receiving window. The transmitter has a state variable VT(S) that holds a sequence number of a protocol data unit (PDU) to be transmitted that is within a transmitting window of the transmitter. The transmitting window has a transmitting window size that indicates the number of sequence numbers spanned by the transmitting window. A beginning value of the transmitting window is held in a state variable VT(A) in the transmitter. A base value that marks a beginning sequence number of the retransmission range is obtained, as well as a head value that marks an ending sequence number of the retransmission range. PDUs with sequence numbers that are sequentially on or after the base value and that are sequentially on or before the head value are capable of retransmission. The head value is given by VT(S)xe2x88x921. If the value (VT(S)+receiving window size) mod 2n does not land within the range of retransmission buffer, i.e. between VT(A) and VT(S)xe2x88x921, inclusively, the base value is given by VT(A). Otherwise, the base value is given by (VT(S)+receiving window size) mod 2n. In addition, if the sum of the reconfigured transmitting window size and the receiving window size is less than or equal to 2n, the base value is given by VT(A).
It is an advantage of the present invention that by expanding the retransmission range, the transmitter may retransmit a greater number of PDUs, which increases the probability of advancing both the receiving and transmitting windows. The efficiency of PDU transportation in the communications system is therefore improved.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.